It is generally known that tungsten values can be readily extracted from tungsten-bearing ores, such as scheelite, by leaching the ore concentrate under pressure with an alkaline solution, such as sodium carbonate, at an elevated temperature, typically about 200.degree. C. It is also generally known that many tungsten-bearing ore concentrates contain relatively large amounts of silicates as impurities which readily dissolve in the alkaline leach solution.
For example, low-grade scheelite concentrates can be effectively leached with sodium carbonate solution under pressure to produce relatively high yields of tungsten values, e.g., about 98-99%. Typically, the leach solution will contain silica in amounts ranging from between about 1 to 3 grams per liter SiO.sub.2 or more. As the solution cools and during subsequent processing, the silica tends to precipitate, causing some tungsten entrainment. Additional silica precipitation occurs upon acidification, a step required in the removal of molybdenum as MoS.sub.3 and in the preparation of the solution for solvent extraction. Some silica is solvent extracted along with the tungsten and is stripped off the organic solvent, along with the tungsten, by aqueous ammonia. Silica precipitates from the strip solution, requiring filtration and causing some tungsten loss. Any silica remaining in the solution at this point accompanies the ammonium tungstate solution to the ammonium paratungstate crystallizer. The silica ties up some of the tungsten and can prevent its crystallization as APT. This requires the recycle of tungsten as mother liquor, thereby increasing the cost per unit of tungsten produced.
By "low-grade" scheelite concentrate as used hereinabove is meant any grade below a commercial grade of concentrate. Typically, these concentrates will contain between about 5 and 50% by weight WO.sub.3.
Tungsten or molybdenum together with nickel or cobalt are commonly used as a catalyst in the hydrocracking of certain petroleum fractions to a more useful product. Generally, the metal catalyst components are present on a zeolite or aluminosilicate substrate. As the catalyst becomes loaded with sulfur, carbonaceous materials and impurity metals, the catalyst efficiency decreases gradually to the point where it is no longer usable from a practical standpoint. This "spent" material, however, still contains the original catalyst metals and becomes a valuable raw material, particularly for the recovery of tungsten.
Another similar source of tungsten is scrap from the original preparation of the catalyst, which for various reasons is unsuitable as catalyst itself. This "scrap" catalyst material generally is not contaminated with sulfur or carbonaceous compounds.
Various attempts have been made to extract tungsten values from spent or scrap catalyst by leaching the catalyst with an alkaline solution, e.g., sodium carbonate. However, these attempts resulted in relatively low yields of tungsten and accordingly they were not considered to be successful.
It has been unexpectedly found that the foregoing difficulties may be effectively avoided by mixing together both the silicate-containing, tungsten-bearing ore concentrate and the spent or scrap catalyst material in appropriate amounts and then leaching or digesting the mixture in an alkaline leach solution, e.g., sodium carbonate, under pressure and at an elevated temperature. The presence of the spent or scrap catalyst material greatly reduces the soluble silica and thus minimizes the subsequent processing problems described hereinabove. This is quite unexpected since the catalyst substrate itself contains significant amounts of silica. Furthermore, it has been surprisingly found that the alkaline pressure leach of the mixture results advantageously in the extraction of extremely high yields of tungsten values that are equal and, in some cases, superior to the yields obtained from the alkaline leach of the tungsten ore concentrate alone.